In theory, the simplest method to prepare aluminum nitride is to heat aluminum metal in the presence of nitrogen; this method is called direct nitridization. In practice, this method is more difficult than it first appears. Since aluminum melts at about 660.degree. C. and the reaction of aluminum and nitrogen begins in general at about 800.degree. C., the aluminum will melt and coalesce into a pool before the reaction begins, which prevents an intimate contact of the reactants. Furthermore, once formed, the aluminum nitride coating acts as a skin or barrier to further reactions. Thus, very low yields are obtained, and the purity of the product aluminum nitride is low because of the large quantities of unreacted aluminum present.
Aluminum nitride can also be obtained by reacting nitrogen with a mixture of aluminum oxide and carbon which has been raised to a high temperature. In order to obtain practically complete conversion of the aluminum oxide to the nitride, it is important that the temperature does not exceed 1800.degree. C., while insuring sufficient nitrogen flow at all times through the entire space filled with the charge of the raw solid material. Any overheating involves volatilizations which impair the efficiency of the reaction, are detrimental to the quality of the end product, and may cause sintering which prevents the continuous operation of the furnace. Lack of nitrogen, even locally, brings about fusion with partial sintering of the charge, so that the completion of the nitriding operation becomes impossible.
Because of the foregoing problems, the commercially available aluminum nitride powder is very expensive. Aluminum nitride is usually produced by nitriding pure aluminum powder (approximately 300 mesh) in a two-step operation. First, the aluminum powder is nitrided for an extended period at about 600.degree. C. to form a coating of aluminum nitride, which prevents coalescense of the molten particles. This is followed by a second treatment in nitrogen at approximately 1200.degree.-1400.degree. C. The product treated in nitrogen at 1200.degree.-1400.degree. C. is unstable to moisture and hydrolyzes very easily. To stabilize the product, additional heat treatment at about 2000.degree. C. is required. In short, the process is slow, tedious, and not cost effective.
U.S. Pat. No. 3,307,908 to Victor Mandorf, Jr. describes a further process for preparing aluminum nitride, which process comprises forming a mixture consisting of finely-divided aluminum metal in the amount of 30-60% by weight and finely-divided carrier material in the amount of 70-40% by weight. The carrier material is selected from the group consisting of aluminum nitride, aluminum fluoride, and mixtures thereof. In this process, a nitriding atmosphere is provided around the mixture which is inert to aluminum nitride and free from oxygen and other materials which interfere with nitriding. Generally, this atmosphere consists of a mixture of nitrogen and ammonia. The mixture is then heated to at least 800.degree. C. while under said atmosphere, thereby nitriding the aluminum to form aluminum nitride.
In U.S. Pat. No. 3,032,398 to Clair, another process for preparing aluminum nitride is disclosed. That process comprises forming particulate material composed of aluminum oxide, carbon and a calcium aluminate binder, continuously passing the particulate material downward by gravity into an elongated externally heated reaction zone wherein the particulate material is heated uniformly to a temperature not in excess of 1750.degree. C., continuously passing a current of nitrogen into said zone, countercurrent to the descended particulate material, to form aluminum nitride. In the process of the Clair patent, an electrically heated shaft furnace is used to conduct the reaction and certain quantities of aluminate of lime, a binder, are required to prevent disintegration during the entire nitriding process. The binder causes crust formation which hinders the proper downward flow of the aluminum oxide-carbon pellets or agglomerates. This system also requires that the reaction temperature be kept at 1750.degree. C. or below. It appears that unless the temperature is kept that low, the reaction product is not easy to grind and cannot be used for its intended purpose.
It is felt that the requirements in the Clair patent are the result of performing the continuous production of aluminum nitride using a countercurrent electric shaft furnace. The electric shaft furnace disclosed by Clair is not a good reaction vessel for conducting this reaction. Clair conducted his process below 1750.degree. C. to avoid sintering of agglomerates, which interferes with the flow of nitrogen to the core of the agglomerate. Good gas particle mixing is absent in this reactor vessel and a local lack of nitrogen is a frequent occurrence. This factor, along with the attendent uneven heating which results since the reaction vessel is heated by resistances from the shell inward, cause the endothermic conversion reaction to stop, and the charge to overheat and fuse together along the walls of the reactor. Once all or part of the charge is fused, operation of the shaft furnace becomes difficult since the charge is no longer free flowing. Additionally, hard crusts of calcium and carbon form in the cooler (1200.degree.-1300.degree. C.) parts of the furnace which also hinder the proper downward flow of the agglomerate. In summary, the electric shaft furnace is plagued by poor gas agglomerate mixing, inadequate thermal uniformity and agglomerate flow problems.
In the past, catalysts, such as calcium fluoride, either were not used or were found to be ineffective. Clair, in U.S. Pat. No. 3,032,398, mentions the use of calcium aluminate as a binder for pelletizing but makes no mention of a catalyst. Sodium fluoride has been used as a catalyst for the direct nitridization of aluminum powder.
Therefore, it is an object of the present invention to provide a process and apparatus for preparing aluminum nitride, which process is capable of a high rate of production and an extremely pure product.
It is a further object of the present invention to provide a continuous process and an apparatus for the preparation of aluminum nitride by carbo-nitridization, which process is capable of a high rate of production and an extremely pure product.
It is another object of the present invention to provide a process and a device for the preparation of aluminum nitride by the carbo-nitridization of alumina, which is capable of producing a high rate of production, and an extremely pure product and cost effective product.
It is another object of the present invention to provide an efficient and cost effective process and a device for the preparation of aluminum nitride by the carbo-nitridization of alumina in which a catalyst allows the reaction to be conducted at temperatures in the range of 1600.degree.-1850.degree. C.